In the manufacture of paper pulp, most notably by the kraft process, wood chips are subjected to the action of an alkaline liquor which attacks the non-cellulose constituents of the chips as, e.g., the fatty and resin acids, and leaches them from the cellulose. The spent liquor of this digestion process is drained from the pulp and is known as the black liquor. The black liquor is usually concentrated and allowed to settle and cool whereupon the sodium soaps of the resin and fatty acids rise to the top and are skimmed off. The skimmed off soaps are known in the trade as crude tall oil soap.
The crude tall oil soap is conventionally reacted with a strong polar acid, most usually sulfuric acid, which converts the crude tall oil soaps to their free acid form. The reaction mixture is allowed to settle and forms a crude tall oil layer, a lignin layer and a sulfate-brine layer. The lignin and sulfate-brine layers are returned to the pulp mill to recover the chemicals for use in the kraft pulping process. The crude tall oil is eventually fractionally distilled to obtain approximately equal portions of rosin acids and fatty acids and to remove the greater portion of the odor and color forming constituents.
Pulp mills have been forced in recent years to upgrade their chemical recovery systems so that there is or will be less and less sulfur loss through the stacks and effluent streams in general and it appears that the various governmental regulations mandating such recovery will be made more stringent in the future. Sulfuric acid acidification of tall oil soap introduces sulfur into the system placing an increased burden on the chemical recovery systems employed. It will accordingly be appreciated that there is a need for a system for acidification of the crude tall oil soap without the use of sulfuric acid. Various attempts for such acidification have been attempted as exemplified, e.g., by U.S. Pat. No. 4,075,188 which employs carbon dioxide in the presence of a water-immiscible solvent, but such attempts have not been commercially successful.
Accordingly, it is the object of this invention to provide a new method for the acidification of tall oil soap without introducing sulfur into the acidification system or products. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.